Recent research conducted at both Lehigh University and Oak Ridge National Laboratory has shown that FeCrAl weld overlays can provide excellent corrosion protection of water-walls in coal fired power plants operating under low NOx firing conditions. These overlays provide several advantages over currently used Ni base alloys. First, they provide significantly better corrosion resistance. Second, they are Fe based (instead of Ni based) and rely primarily on Al (rather than Cr) as a preferential oxidizing element. As a result, they are significantly cheaper (~ $5-10/lb compared to $20-30/lb). Lastly, the Al and Cr are uniformly distributed on a microscopic scale throughout the coating. In contrast, commercial Ni base overlays exhibit microsegregation that causes preferential corrosion and corrosion-fatigue cracking. The FeCrAl alloys should be immune to this problem since they do not exhibit microsegregation. However, use of FeCrAl alloys by industry is currently limited due to their susceptibility to hydrogen cracking during welding.

Recent weldability studies using the gas tungsten arc welding (GTAW) process has shown that the hydrogen cracking resistance of FeCrAl alloys can be significantly improved by the addition of second phase particles that act as hydrogen trapping sites. In fact, crack-free deposits of ordered intermetallic alloys with ~ 15wt% Al, which were previously considered non-weldable, were successfully prepared when second phase carbide particles were present in the fusion zone. Previous weldability results showed that only ~ 10wt% Al could be tolerated before the onset of cracking when carbides were not present. Although these results are promising, the GTAW process is not used by industry because of the low deposition rates. Industry uses the gas metal arc welding (GMAW) process for this application. Unfortunately, it is well known that this process leads to higher levels of hydrogen in the weld, and GMA welds are therefore more susceptible to hydrogen cracking. Recent results conducted at Lehigh on FeCrAl weld overlay alloys have confirmed this trend. Weld overlays prepared with the GMAW process could only tolerate ~ 7 wt% Al before cracking occurred. Results from corrosion tests have shown that this Al level is too low to provide adequate corrosion protection. Therefore, the hydrogen cracking resistance of FeCrAl weld overlays needs to be improved before these alloys can be used for corrosion protection on a large scale basis by industry.

The objective of this work is to develop methods for improving the hydrogen cracking resistance of FeCrAl weld overlays so that they can be used for corrosion protection in power generation applications. A preliminary review of the literature shows that there are two promising approaches for improving the hydrogen cracking resistance of FeCrAl weld overlays: 1) Controlled addition of hydrogen trapping sites such as carbides and oxides to reduce the level of diffusible hydrogen in the weld, and 2) Addition of constituents that react with hydrogen in the arc to produce insoluble products in the overlay. The effectiveness of second phase particles to trap hydrogen depends on several key factors, such as the second phase type (i.e., carbide or oxide), crystal structure, and binding energy. Similarly, the effectiveness of various constituents to react with hydrogen in the arc varies by the constituent type. In addition, it will be confirmed that any additions made to the weld overlay, for controlling hydrogen cracking, do not adversely affect the corrosion resistance.

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